Once you receive your physical One card, you will have an option to add it to Apple Pay. Here is a link from Apple with information about how to add a card to Apple Pay.
Screenshots provided to guide you through Apple Wallet:
Step 1: Tap the “+” sign in the top right corner.
Step 2: Select ‘Credit or Debit Card.’
Step 3: Continue.
Step 4: You’ll be prompted to take a photo of the side of your One card that contains your account number.